Hot melt adhesive guns are known for application of a melted thermoplastic adhesive onto a work surface on which the adhesive solidifies to form a requisite bond. One class of hot melt gun employs a cartridge of normally solid thermoplastic adhesive contained within a chamber and in which the cartridge or portion thereof is heated and melted and by pneumatic action urged through a nozzle of the gun for dispensing onto a work surface. A gun of this type is shown in U.S. Pat. No. 3,764,045 assigned to the assignee of this invention. In other known hot melt guns, a piston is disposed within a chamber and is pneumatically driven along the length of the chamber to urge the adhesive cartridge forward and force the melted adhesive through the gun nozzle. This latter type of gun is shown, for example, in U.S. Pat. No. 3,587,930, where an adhesive chamber is formed by registered first and second cavities with a forward cavity being heated to provide a reservoir of molten adhesive and a rearward cavity being sufficiently cooler to maintain the adhesive in solid form and including a pneumatically-driven piston in sliding engagement with the rearward cavity wall. Another gun of this latter type is shown in U.S. Pat. No. 3,877,610 assigned to the assignee of this invention, and in which a stationary piston is disposed within a cylindrical member which is slidable with respect to the stationary piston upon pneumatic actuation. The cylindrical member is mounted for slidable movement within the adhesive chamber to urge the adhesive forward for melting and dispensing.
Many hot melt guns of the type cited above employ needle valves to dispense the thermoplastic material from the nozzle. A gun of this type is shown, for example, in the aforesaid U.S. Pat. No. 3,587,930, which employs a valve housing and a movable needle which is manually actuable. Needle valves of this nature are subject to carbon buildup on the end of the needle which retards the motion of the needle and which in turn either slows down or completely prevents the closing of the needle valve. In addition, this particular type of valve requires the use of seals to prevent the leakage of glue past the needle when the valve is closed. Of necessity, these seals must be internal and immersed in the adhesive, which produces gumming problems and problems with replacement. Generally, these types of needle valve nozzles use a gland which is adjustable, and the adjustment of this gland poses a problem in that if the gland is adjusted too tightly, the needle does not move freely enough and the glue is not allowed to flow. If the gland is too loose, leaking of the glue occurs. Other types of needle vales employ pneumatic means to actuate them as opposed to the manual device employed above.